1. Field of the Invention
This invention relates to a method and apparatus for making absolute distance or ranging measurements using Fresnel diffraction. The invention employs a source of electromagnetic radiation having a known wavelength or wavelength distribution, which sends a beam of electromagnetic radiation through an object which causes it to be split (hereinafter referred to as a "beamsplitter"), and then to a target. The beam is reflected from the target onto a screen containing an aperture spaced a known distance from the beamsplitter. The aperture is sized so as to produce a Fresnel diffraction pattern. A portion of the beam travels through the aperture to a detector, spaced a known distance from the screen. The detector detects the central intensity of the beam. The distance from the object which causes the beam to be split to the target can then be calculated based upon the known wavelength, aperture radius, beam intensity, and distance from the detector to the screen. Several apparatus embodiments are disclosed for practicing the method embodiments of the present invention.
2. Description of the Prior Art
Prior art interferometers are unable to measure absolute distances, are costly. Interferometers are limited to relative distance measurements, rather than absolute distance measurements. Interferometers usually require a specularly reflective target surface from which light is reflected to form an interference pattern.
Other prior art methods of making distance measurements include triangularization and digital detection. Such methods have a limited range in which micron accuracy is available. This limited range is normally less than five millimeters. Such devices also have a limited accuracy, on the order of greater than one micron for ranges in excess of five millimeters. Such devices further require frequent calibration.
The present invention provides a method and apparatus for measuring absolute distances over long ranges with high precision. The present invention can be operated in either a continuous or pulsed mode, requires minimal optical alignment, provides rapid data acquisition and is capable of making measurements over a wide variety of target surface qualities including very diffuse and highly reflective surfaces.
The present invention provides extremely high accuracy in measuring absolute distances. The present invention is capable of making absolute distance measurements on the order of magnitude of a meter, with an accuracy on the order of magnitude of an 0.1 micrometers.